Problem with Fresnel Artifacts in PBR Shader
For the glitchy normals when not using a bump/normal map: When you have a loose shader node with no connection to the normal inputs, Cycles defaults to the mesh normal to avoid shading artifacts. However, when the shader normal is connected to a group input, this doesn't happen. So the shader evaluation normal instead gets initialized to whatever the group input socket default was. You can fix this by connecting the Geometry node's "normal" output to where your normal map would go (if you had one). This just explicitly tells Cycles to use the mesh normal.